1. Field of the Invention
The present invention relates to an optical element and an optical system using the same and, more particularly, to an optical system suitable for a silver halide camera, video camera, still video camera, or copying machine, which uses an optical element having a plurality of reflecting surfaces to form an object image on a predetermined plane and reduce the size of the entire optical system.
2. Related Background Art
Various photographing optical systems using the reflecting surfaces of concave mirrors or convex mirrors have been conventionally proposed. FIG. 13 is a schematic view of a so-called mirror optical system (reflecting optical system) comprising one concave mirror and one convex mirror.
In the mirror optical system shown in FIG. 13, an object light beam 124 coming from an object is reflected by a concave mirror 121, travels to the object side while being focused, and is reflected by a convex mirror 122, thereby forming an image on an image plane 123.
This mirror optical system is based on the arrangement of a so-called Cassegrain reflecting telescope. This system aims at shortening the total length of the optical system by deflecting the optical path of a telephoto lens system, which comprises refracting lenses and has a large total lens length, using two reflecting mirrors opposing each other.
For an object lens system of a telescope as well, a number of schemes for reducing the total length of the optical system using a plurality of reflecting mirrors are known in addition to the Cassegrain scheme for the same reason.
In this way, conventionally, a compact mirror optical system is obtained by using reflecting mirrors in place of photographing lenses with a large total lens length to efficiently deflect the optical path.
However, generally, in the mirror optical system such as a Cassegrain reflecting telescope, the object light beam is partially eclipsed by the convex mirror 122. This problem is posed because the convex mirror 122 is inserted in the path of the object light beam 124.
To solve this problem, a mirror optical system which prevents a portion of the optical system from blocking the path of the object light beam 124, i.e., separates a principal ray 126 of the light beam from an optical axis 125 by decentering reflecting mirrors has also been proposed.
FIG. 14 is a schematic view of a mirror optical system disclosed in an U.S. Pat. No. 3,674,334. This system solves the problem of eclipse by separating the principal ray of an object light beam from the optical axis. The mirror optical system shown in FIG. 14 comprises a concave mirror 131, a convex mirror 132, and a concave mirror 133 in the order the light beam passes through. These mirrors are originally rotationally symmetric with respect to an optical axis 134, as is indicated by alternate long and two-dashed lines in FIG. 14. A principal ray 136 of an object light beam 135 is separated from the optical axis 134 by using only a portion of the concave mirror 131 above the optical axis 134, only a portion of the convex mirror 132 below the optical axis 134, and only a portion of the concave mirror 133 below the optical axis 134, thereby constructing an optical system free from eclipse of the object light beam 135.
FIG. 15 is a schematic view of a mirror optical system disclosed in U.S. Pat. No. 5,063,586. In the mirror optical system shown in FIG. 15, the central axes of reflecting mirrors are decentered from the optical axis to separate the principal ray of an object light beam from the optical axis, thereby solving the above problem.
Referring to FIG. 15, an axis perpendicular to an object surface 141 is defined as an optical axis 147. The central coordinates and central axes (a central axis is formed by connecting the center of a reflecting surface to the center of curvature of the surface) 142A, 143A, 144A, and 145A of the reflecting surfaces of a convex mirror 142, a concave mirror 143, a convex mirror 144, and a concave mirror 145, which are located in the order the light beam passes through, are decentered from the optical axis 147. In FIG. 15, by appropriately setting the decentering amounts and radii of curvature of the surfaces, the reflecting mirrors are prevented from eclipsing an object light beam 148, so an object image is efficiently formed on an imaging plane 146.
U.S. Pat. Nos. 4,737,021 or 4,265,510 also discloses an arrangement for avoiding eclipse partially using reflecting mirrors rotationally symmetric with respect to the optical axis or an arrangement for avoiding eclipse by decentering the central axes of reflecting mirrors from the optical axis.
As described above, when the reflecting mirrors constituting the mirror optical system are decentered, eclipse of the object light beam can be prevented. However, the reflecting mirrors must be set with different decentering amounts. This complicates the structure to which the reflecting mirrors are attached and also makes it difficult to ensure given attachment precision.
To solve this problem, a method of avoiding assembly errors of optical components in an assembly by forming the mirror system as one block has been proposed.
Conventionally, as a structure in which a number of reflecting surfaces form one block, there is an optical prism such as a pentagonal roof prism or Porro prism used in the viewfinder system of a camera, or an optical prism such as a color separation prism for separating a light beam from a photographing lens into, e.g., three color light components: red, green, and blue light components, and forming an object image based on each color light component on the surface of a corresponding image sensing element.
In these prisms, a plurality of reflecting surfaces are integrally formed, and have a precise relative positional relationship, so position adjustment of the reflecting surfaces is unnecessary. However, the main function of these prisms is to change the direction the light beam travels to invert the image, and each reflecting surface is flat.
On the other hand, an optical system using a prism whose reflecting surface has a curvature is also known.
FIG. 16 is a schematic view of an observation optical system disclosed in U.S. Pat. No. 4,775,217. This observation optical system observes the landscape and simultaneously observes an image displayed on an information display device overlapping the landscape.
In this observation optical system, a display light beam 165 emerging from an image displayed on an information display device 161 is reflected toward the object side by a surface 162 and enters a half mirror surface 163 formed from a concave surface. The display light beam 165 is reflected by the half mirror surface 163 and then converted into a nearly collimated light beam by the refracting power of the concave surface 163. The display light beam 165 is refracted and transmitted through the surface 162 to form an enlarged virtual image of the displayed image, and simultaneously enters a pupil 164 of the observer to allow him/her to see the display image.
An object light beam 166 from the object enters a surface 167 almost parallel to the reflecting surface 162, is refracted, and reaches the concave half mirror surface 163. The half mirror surface 163 is coated with a semi-transparent film. Some components of the object light beam 166 pass through the concave surface 163, are refracted and transmitted through the surface 162, and enter the pupil 164 of the observer. With this arrangement, the observer sees the displayed image that overlaps the landscape.
FIG. 17 is a schematic view of an observation optical system disclosed in Japanese Laid-Open Pat. Application No. 2-297516. This observation optical system also serves to observe an external landscape and simultaneously observes an image displayed on an information display device overlapping the landscape.
In this observation optical system, a display light beam 174 emerging from an information display device 170 is transmitted through a flat surface 177 of a prism Pa and enters the prism Pa and then a parabolic reflecting surface 171. The display light beam 174 is reflected by the reflecting surface 171 and forms an image on a focal-plane 176 as a focused light beam. The display light beam 174 reflected by the reflecting surface 171 is totally reflected by two flat surfaces 177 and 178 parallel to each other of the prism Pa and reaches the focal plane 176. With this arrangement, a low-profile optical system is achieved.
The display light beam 174 leaving the focal plane 176 as a divergent light beam is totally reflected by the flat surfaces 177 and 178 and enters a half mirror 172 having a parabolic surface. The display light beam 174 is reflected by this half mirror surface 172 and simultaneously forms an enlarged virtual image of the displayed image by the refracting power. At the same time, the display light beam 174 is transmitted through the surface 177 as a nearly collimated light beam and enters a pupil 173 of the observer, thereby allowing an observer to see the displayed image.
On the other hand, an object light beam 175 from the outside passes through a surface 178b of a prism Pb, the half mirror 172 having a parabolic surface, and the surface 177, and enters the pupil 173 of the observer. With this arrangement, the observer sees the displayed image that overlaps the landscape.
As examples of an optical system using an optical element for the reflecting surface of a prism, there are optical heads for optical pickups disclosed in, e.g., Japanese Laid-Open Pat. Application Nos. 5-12704 and 6-139612. In these optical systems, after light from a semiconductor laser is reflected by a Fresnel or hologram surface, an image is formed on a disk surface, and light reflected by the disk is guided to a detector.
To solve the above problems, the present applicant of the basic application filed in Japan has made a proposal aimed at providing a reflecting optical system which uses a plurality of optical elements, each of which has a plurality of curved or flat surfaces that are integrally formed, to reduce the size of the entire mirror optical system and also relax the arrangement precision (assembly precision) of reflecting mirrors, which is likely to be high in the mirror optical system, and an image pick-up apparatus using the reflecting optical system.
Referring to FIG. 18, in an optical element 51, a plurality of reflecting surfaces having curvatures are integrally formed. The optical element 51 has, sequentially from the object side, a concave refracting surface R2 having a negative refracting power, five reflecting surfaces of a concave mirror R3, a convex mirror R4, a concave mirror R5, a convex mirror R6, and a concave mirror R7, and a convex refracting surface R8. Incident and exit reference axes of the optical element 51 are made substantially parallel but opposite in direction. The optical system also has an optical correction plate 52 such as a quartz low-pass filter or an infrared cut filter, an image pick-up element surface 53 of a CCD or the like, a stop 54 inserted on the object side of the optical element 51, and a reference axis 55 of the photographing optical system.
The image forming relationship in FIG. 18 will be described. Light 56 from an object is limited in its light amount by the stop 54 and then enters the concave refracting surface R2 of the optical element 51.
The object light 56 incident on the concave refracting surface is converted into divergent light by the power of the concave refracting surface R2 and reflected by the concave mirror R3. At the same time, a primary object image is formed on an intermediate imaging plane N1 by the power of the concave mirror.
Having formed a primary image on the intermediate imaging plane N1, the object light 56 is repeatedly reflected by the convex mirror R4, concave mirror R5, convex mirror R6, and concave mirror R7 while being influenced by the power of each reflecting mirror, and reaches the convex refracting surface R8. The object light 56 refracted by the power of the convex refracting surface R8 forms an object image on the image pick-up element surface 53.
As described above, the optical element 51 functions as a lens unit which attains a desired optical performance and a positive power as a whole while repeating refraction by incident/exit surfaces and reflection by the plurality of reflecting mirrors having curvatures.
In recent years, extensive studies have been made for lenses having refractive index profile (to be referred to as "GRIN lenses" hereinafter), and some gradient index lenses have already been put into practical use. With the advent of GRIN lenses, optical designers can set new parameters and use GRIN lenses as effective means for correcting aberrations such as an aspherical surface effect. A GRIN lens has the following advantages.
It is compact and lightweight. PA1 It can be tightly cemented to another optical element. PA1 It can have an arbitrary focal length. PA1 It can form an achromatic lens by itself. PA1 It can form a high-dispersion prism.
As a method of manufacturing a GRIN lens, a method of forming a refractive index profile (distribution) by ion exchange, and a method using a ceramics manufacturing method called the sol-gel method are known. In the ion exchange method, preform glass is brought into contact with a molten salt containing ions to be diffused so as to form an ion distribution from the surface toward the inside in the preform glass member by ion exchange from the surface, thereby forming a refractive index profile. An example of the ion exchange method is disclosed in, e.g., Japanese Laid-Open Patent Application No. 62-47826. In the sol-gel method, materials for preform glass in a state of liquid are mixed to prepare a gel. The wet gel is dried and subjected to ion exchange in an ionic solution to form a refractive index profile in the gel. This gel is dried and calcined at a high temperature. An example of the sol-gel method is disclosed in, e.g., Japanese Laid-Open Patent Application No. 1-51335.
In the ion exchange method, however, the profile is determined in the process of performing ion exchange from the surface, so a refractive index profile is formed only near the surface. In addition, since the types of diffusible ions are limited, large differences of refractive indices in profile cannot be set.
In the sol-gel method, the shrinkage factor in high-temperature calcination is as high as several tens of percent, and crack occurs in the case of producing a large lens. In addition, since the degree of freedom in selecting materials is limited, large differences of refractive indices in profile cannot be obtained.
Japanese Laid-Open Patent Application No. 9-65246 discloses a method of using a decentered prism and a gradient index lens in an image display apparatus to generate aberration of opposite sign to that generated on the transmission surface of the decentered prism, thereby correcting chromatic aberration.
In all the mirror optical systems having decentered mirrors, which are disclosed in U.S. Pat. Nos. 3,674,334, 5,063,586, and 4,265,510, the reflecting mirrors have different decentering amounts. This arrangement considerably complicates the structure to which the reflecting mirrors are attached and also makes it very hard to ensure required attachment precision.
Both the observation optical systems disclosed in U.S. Pat. No. 4,775,217 and Japanese Laid-Open Patent Application No. 2-297516 mainly aim at obtaining the pupil imaging function for efficiently guiding the image displayed on the information display device spaced apart from the pupil of the observer to the pupil of the observer and changing the traveling direction of light. These prior disclosures do not directly disclose any techniques of positively correcting aberration using reflection surfaces having curvatures.
Both the optical systems for optical pickups, which are disclosed in Japanese Laid-Open Patent Application Nos. 5-12704 and 6-139612, are limited to use as detection optical systems and do not satisfy the imaging performance of a photographing optical system and, more specifically, a photographing apparatus using an area image pick-up element such as a CCD.
To correct aberration by a single GRIN lens, a large refractive-index difference .DELTA.n must be set. This is very difficult in terms of manufacturing the material required.
The image display apparatus disclosed in Japanese Laid-Open Patent Application No. 9-65246 requires two components: a decentered prism and gradient index lens. This is relatively ineffective in reducing the apparatus size or facilitating assembly.